While you could use the support for them to split human-managed code across separate source files, that would be a horrible practice that I've never encountered in the wild, even in .NET shops with otherwise-atrocious practices and code quality.
To continue that type of advice, some say "#region/#endregion" is another language feature that's intended for code generators so that the IDE can collapse specific lines of code and hide it from view. Programmers should not be hand-coding "#region" themselves. That said, there is debate on that: https://softwareengineering.stackexchange.com/questions/5308...
Where does the absolutism in the tech industry come from? We are a bunch of individuals who have individual experience and then try to form a view of the world that satisfies our experiences. What about the experiences you haven't had or conceived of? We are constantly rewriting the rules in our head to fit the new experiences we have every day to make sure we are right all of the time. Surely, our current world views are not complete or we would have no room to grow.
Still, I'll take your comment under advisement in case my classes are big, poorly designed non-Tries.
I have a minor doubt that I won't like the additional level of nesting incurred, but I'll attempt it regardless.
Like others have said, it makes working with generated code easier.
The namespace keyword is used to declare a scope that contains a set of related objects. You can use a namespace to organize code elements and to create globally unique types.
And about partial classes:
There are several situations when splitting a class definition is desirable:
When working on large projects, spreading a class over separate files enables multiple programmers to work on it at the same time.
When working with automatically generated source, code can be added to the class without having to recreate the source file. Visual Studio uses this approach when it creates Windows Forms, Web service wrapper code, and so on. You can create code that uses these classes without having to modify the file created by Visual Studio.
As you can see partial classes are not the right tool to organise code.
Also, "regardless of the use case" ? I barely know how to respond to that. Have a little imagination.
(1) that's an exaggeration; MFC used comments to identify sections that it owned in the source, but in my (limited) experience, there typically were zillions of ways to make changes to your code, but if you didn't use the one Microsoft picked as _the_ way (and which they didn't push into your face in the IDE), you were in for heaps of problems.